Ecological Modelling, 38 (1987) 243-256 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 243 A RECENT INCREASE IN JELLYFISH POPULATIONS: A PREDATOR-PREY MODEL AND ITS IMPLICATIONS TARZAN LEGOVIC Center for Marine Research, 'Rudjer Bo#kovir' Institute, 41001 Zagreb (Yugoslavia) (Accepted 2 October 1986) ABSTRACT Legovir, T., 1987. A recent increase in jellyfish populations: a predator-prey model and its implications. Ecol. Modelling, 38: 243-256. Factors that might have caused a recent increase of jellyfish populations in the Mediter- ranean sea are investigated using a simple predator-prey model. Higher nutrient inflow causes an increase in the steady state of a jellyfish population and no change in the steady state of jellyfish prey. Qualitatively the same change is obtained if the population of predators competing for jellyfish prey is decreased. A decrease of predators on the jellyfish population causes an increase in jellyfish prey. Sufficiently high nutrient enrichment, or a decrease in jellyfish competitors or predators, may cause the appearance of a stable limit cycle, i.e., asymptotically periodic fluctuations of both the jellyfish population and its prey. If persistent periodic fluctuations of jellyfish and their prey are a natural phenomenon, then the above ecological changes will increase the amplitude of the periodic fluctuations. INTRODUCTION Since 1977 it has been observed that populations of jellyfish Pelagia noctiluca (Forsk~l) have increased considerably in the Adriatic Sea (Malej, 1980) and the whole of the Northern Mediterranean. The phenomenon has aroused much interest in the Mediterranean countries because of its negative effects on tourism and fisheries. The following three hypotheses have been put forth to account for the observed massive appearance of jellyfish: (1) Nutrient enrichment induces growth of jellyfish population since the jellyfish are able to indirectly utilize the nutrient pool (Wilkerson and Dugdale, 1983) (2) An unusually strong entrance (ingression) of Mediterranean waters brings to and maintains in the Adriatic Sea a higher population (Vu~etid, 1983). Field measurements have shown that indeed during the recent ingres- 0304-3800/87/$03.50 © 1987 Elsevier Science Pul~lishers B.V.